Liberal Democrat leader Nick Clegg say he finds it “bewildering” that there are members of a “progressive” Labour Party campaigning against the Alternative Vote.

Mr Clegg spoke out against senior Labour members who were backing the No Campaign and said the current first-past-the-post system was “regressive” and had become “the play thing of vested interests”.

Mr Clegg said: “The Labour Party has a very proud tradition of being a progressive party and I think it is a good thing that Ed Miliband and other figures in the Labour Party want to come and make the case. I think it's bewildering that there are people in the Labour Party who want to cling to the old system.

“I guess they like it because they know how the system works to their advantage. I'm not sure I'm the best advocate for the Labour Party or that they are going to listen to me.”

Speaking directly to a Labour supporter in the audience, Mr Clegg said that he could not understand how a progressive party could oppose changing the current voting system: “I would say to you as a Labour supporter, as to someone who I assume has a progressive turn of mind, I just cannot understand how someone of a progressive turn of mind thinks it is OK to have a political system that has become the play thing of vested interests, which has happened with first past the post,” Mr Clegg told her.

“That is very regressive. When cliques of the old established parties in particular like the system because it allows them to be returned over and over again and only dare to get 30% of the vote. That cannot be right.”